It is certainly true that the Supremacy Clause gives the federal courts a rationale to allow federal law to displace state law. But it is a doctrine that is selectively invoked and interpreted. As for Gardner, the case turns on one’s political perception of the United States. The original conception was not full sovereignty, but an agent of the states with limited and enumerated powers. Under this conception, one might easily hold that the United States, when it comes into possession of property other than by purchase from a state with some sort of state relinquishment of state sovereignty, holds the land as agent for the states, or in trust for them, to be gotten rid of in a reasonably expeditious manner. Once one conceives of the United States as a full sovereign, with numerous powers inherent to sovereignty (how we wound up with printing press money, by they way), then one takes a different view.
“Once one conceives of the United States as a full sovereign, with numerous powers inherent to sovereignty (how we wound up with printing press money, by they way), then one takes a different view.”
That kind of sounds like the US Constitution, as opposed to the confederacy that existed before the Constitution.
https://en.wikipedia.org/wiki/Articles_of_Confederation